This invention is concerned with filters used to provide a very clean environment at work stations, e.g., at assembly stations for electronic circuits and the like.
In order to provide a very clean atmosphere at an assembly table a very clean flow of air is directed vertically downwards towards the table. In this way an air barrier is obtained preventing surrounding dust from reaching the workpieces. An advantage with this arrangement is that not even the person carrying out the work will contribute to the contamination of the workpiece. In general these filters are of the so-called "absolute" type in that they allow passage of virtually no contaminants. These filters are made by a rectangular zig-zag folding of a filter sheet structure with aluminum distance elements or spacers placed between the folds, the distance elements being in the form of zig-zag bent aluminum strips. As is apparent already from this brief description of the filters, they are very expensive, due to both the materials involved and the great amount of work necessary for fabrication.
Attempts have therefore been made to produce these filters without the expensive aluminum distance elements. It has for instance been suggested to use simple filters consisting of only zig-zag folded paper elements. Regretfully this has not turned out to be a very good solution. Filters produced in this way can cope with a necessary amount of air and they also provide a sufficiently clean air, but they do not give the required even flow of air leaving the filter. Instead, they result in a turbulent swirling flow of a type which totally destroys the clean environment at the work piece. Surrounding dust can easily be trapped in the swirls and be swept to the workpiece.
The above-described more expensive absolute filters including aluminum distance elements, however, produce an extremely even flow. The reason for this is that the air is very exactly directed through the filter by the aluminum strips which assuredly provide a greater number of parallel channels. The less expensive filters, whether they are of the absolute kind or not, do not however exhibit the same degree of precision in guiding the air which is streaming through, and this lack of precision results in flow variations.
In view of the above comments, it is an object of the invention to provide a filter that is less costly to fabricate, but is still usable in situations where a very even air flow is necessary from the filter for instance at work stations where a very great degree of air purity must be achieved.